How you can trust whether PISA participating students are randomly chosen ? If not randomly chosen therefore how can results be valid ?
I cannot speak to an overall system, or level of confidence for all countries. I can only imagine there is a wide variety of motivations and interests at play. My experience in this area is limited to Costa Rica, where I have been teaching in secondary schools since 2009. Only once has a school I was working in been part of the PISA. I have worked at three private schools during that time, and two of them have consistently scored high to very high compared to the national average (based on school-leaving bachillerato exams and scores for the three universities that use their own, highly competitive, entrance exams). The PISA test we administered (I believe it was either in 2015 or 2016) came as a directive from the ministry of education (MEP) with a list of the 16 students who were to take it. I could be wrong on the number of students, but I am certain it was not the entire grade, and that the names of the students were provided to the school from MEP. Based on the fact that the students involved was part of the information provided to us from MEP and the fact that (in my, albeit, very limited experience) a high performing school was only chosen once, I would tend to think that they are indeed randomly chosen. Also, I remember the students asking why they needed to try on the test as there were "no points" on their grade for participation.
If it is of any use to you, it would seem to me that, at least in the one I witnessed, everything was on the up and up. I am not sure how you could know, for certain, about all countries/schools though.
International secondary school PISA scores may be less representative if they primarily sample the brightest students from top schools, potentially skewing results and not reflecting the broader student population's performance accurately.
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